Method and apparatus for measuring passively range and bearing

ABSTRACT

In an aircraft collision avoidance system, a method and apparatus for determining passively the slant range and bearing angle to another transponder equipped aircraft within a selectable proximity to an aircraft (own aircraft) equipped with the collision avoidance system in which the azimuthal lines of position from interrogating standard secondary surveillance radar ground stations to both aircraft and the times of arrival at the own aircraft of the transponder replies of the other aircraft are determined. From such data, the bearing angles to the actual and imaginary locations of the transponder equipped aircraft are calculated and then supplied to a truth table which identifies the bearing angle to the actual location of the transponder equipped aircraft. The slant range to the transponder equipped aircraft is then computed in accordance with a predetermined relationship between the time of arrival of one of the transponder replies and the bearing angle.

O United States Patent [I 1 [111 3,895,382 Litchford July 15, 1975METHOD AND APPARATUS FOR Primary ExaminerMalcolm F. Hubler MEASURINGPASSIVELY RANGE AND Attorney, Agent, or FirmBrumbaugh, Graves, mNGDonohue & Raymond [75] Inventor: George B. Litchford, Northport,

N.Y. [57] ABSTRACT [73] Assignee: Litchstreet Co., Northport, N.Y. In anaircraft collision avoidance system, a method [22] Filed: Jan 31 1974and apparatus for deterrnlmng passively the slant range and bearingangle to another transponder [21] Appl. No.: 438,297 equipped aircraftwithin a selectable proximity to an aircraft (own aircraft) equippedwith the collision 52 us. Cl. 343/65 LC; 343/112 CA avoidance Systemwhich azimuhal tion from interrogating standard secondary surveil- [51]Int. Cl. G015 5/02; GOls 9/02 lance radar ground stations to bothaircraft and the [58] Field of Search 343/65 R. 6.5 LC. 112 CA times ofarrival at the own aircraft of the transponder replies of the otheraircraft are determined. From such [56] Relerences cued data, thebearing angles to the actual and imaginary UNITED STATES PATENTSlocations of the transponder equipped aircraft are cal- ,772 l/l965Bagnall. Jr. et al 343/66 LC X culated and then supplied to a truthtable which iden- 3,3l2,97l 4/1967 Gehman 343/65 L tifies the bearingangle to the actual location of the 3,336,59l 8/[967 Michmk et al.343/65 LC transponder equipped aircraft. The slant range to the 52 3 gtransponder equipped aircraft is then computed in ac- 'g g'l3 2119712:11; 343,6 5 R cordance with a predetermined relationship between3:626:41] l2/l97l Lirchr ra iljun ij..,...I: 343/63 x helime of of oneof "ansponder replies Reese and the bearing angle.

19 Claims, 12 Drawing Figures OWN STATION J WAVE FRONT SHEET 1 FIG. [.4

( OTHER l Rcos6{ e R B (11,0) X OWN STATION--J I WAVE FRONT I FIG. l5

I WAVE FRONT T PATFTHTEQ JUL 15 ms SHEET 2 MAGNETIC WAVE FRONT NORTH o|+cos(9 INTERSECTION POINT a x (-T1,0) (T 0) mnznsecnou POINT 2 "MmeWAVE FRONT T P A TEIITEDJUL I 5 I975 SHEET 5 T 4 r a,

COMB.

50 CKT.

A 50B\ coma Az PA CKT 54A7 52A) 5 31 e 52B) e SIGN SIGN SIGN SIGNDETECTOR DETECTOR DETECTOR DETECTOR 6A IBO= 6 |a0=+ 9 |e0= 6 |ao= 9 I80=G l80= 9 I8O= 9 I80= I I 5bA COMP. COMP.

\ COMP. j COMP.

I as I I, I 6 A 608 A1 0 B| T TRUTH GATE TRUTH GATE v 628 03 \e I TRANS.GATE TRANS. GATE 64 L 1 v 1 I +605 9 comame ,70 cx, WITH 6 OR 65,

RANGE RANGE BEARING RATE I N E mu INDICATOR 'NDIPATOR PMEMTEUJUL 15 msSHEET m mwm METHOD AND APPARATUS FOR MEASURING PASSIVELY RANGE ANDBEARING CROSS-REFERENCE TO RELATED PATENTS AND APPLICATIONS Thisapplication is related to the following United States patents and patentapplications:

I. US Pat. No. 3,626,4ll of George B. Litchford,

issued Dec. 7, 1971. 2. US. Pat. No. 3,735,408 of George B. Litchford,

issued May 22, I973. 3. US. Pat. No. 3,757,324 of George B. Litchford,

issued Sept. 4, I973. 4. US. Pat. Application Ser. No. 317,810 of GeorgeB. Litchford, filed Dec. 22, I972. 5. US. Pat. Application Ser. No.345,432 of George B. Litchford, filed Mar. 27, I973. 6. US. Pat.Application Ser. No. 37 l ,839 of George B. Litchford, filed June 20,1973. 7. US. Pat. Application Ser. No. 371,883 of George B. Litchford,filed June 20, I973.

BACKGROUND OF THE INVENTION The present invention pertains toradiolocation of mobile vehicles within the coverage of at least twoscanning radars of a standard secondary radar system.

More particularly, the invention concerns a collisionavoidance/proximity warning system, capable of determining the slantrange and relative bearing to a nearby mobile vehicle, that is based onsignals emitted by secondary radars, such as the National Air TrafficControl Radar Beacon System (ATCRBS) and the International CivilAviation Organization (ICAO) Secondary Surveillance Radar System.

Major airports and way points are presently equipped with secondarysurveillance radar (SSR) adapted to cooperate with transponders carriedon aircraft to discriminate against interference and ground clutter andto provide for automatic transmission of identification and other data,such as altitude, from the aircraft to the ground-based radar. A trafficcontroller observing the radar display directs the pilots of theinvolved aircraft by radio, usually with voice communication, so as tomaintain or restore safe separations between aircraft. Such a system oftraffic control and separation assurance is limited in capabilitybecause each aircraft must be dealt with individually and requires itsshare of the controller's time and attention and its share of theavailable radio spectrum. When traffic is heavy, takeoffs and landingsare delayed, and the possibility of col lision increases.

The potential for disastrous mid-air collisions has become so pronouncedthat numberous inter-aircraft cooperative proximity warning systems havebeen proposed. Those more prominently under study or development at thistime involve frequent or quasicontinuous exchange of signals between allcooperative aircraft within the region ofinterest and make no provisionfor non-cooperating aircraft. The required airborne equipment would bebulky and expensive, use more of the already crowded radio spectrum andwould be generally independent of other needed and existing equipment,such as transponders. Another drawback of some of the proposed systemsis that they provide only relative positional information, withoutground reference but in effect with respect to a randomly floatingreference.

Nearly all the disadvantages of these proposed systems may be overcomeby providing aircraft with a collision avoidance system based on thealready existing secondary surveillance radar system. Particularly ifthe identity, altitude, range, and bearing of all aircraft in theproximity of one's own aircraft can be obtained entirely passively i.e.,by merely listening" to the ground signals and transponder replies ofnearby aircraft to interrogations of radar ground stations it will bepossible to provide an effective warning in time to avoid collisionswithout major outlay for an entirely new system and without utilizing anadditional portion of the radio spectrum.

The need for such a system has been recognized in Congress, asillustrated by the proposed legislation HR. 9758 recently introduced inthe House. This legislation provides as follows:

AMENDMENTS TO THE FEDERAL AVIATION ACT OF I958 See.4a. Section 10! ofthe Federal Aviation Act of I958 is amended by inserting at the endthereof the following:

(37) Collision avoidance system means an all weather cooperative systemon an aircraft which is compatible with but independent of ground-basedair traffic control systems and can detect all other aircraftrepresenting a potential collision threat and, if necessary, indicate tothe pilot a safe evasion maneuver.

b. Section 601, of the Federal Aviation Act of 1958 is amended by addingat the end thereof the following new subsection:

COLLISION AVOIDANCE SYSTEMS d. (I). Minimum standards pursuant to thissection shall include the requiremment that within a reasonable timeafter its enactment (A) a collision avoidance system shall be installedon any aircraft which is operated by an air carrier or a supplementalair carrier, and has a maximum certificated takeoff weight in excess ofsixty thousand pounds;

(B) a collision avoidance system shall be installed on all publicaircraft;

(C) a collision avoidance system shall be installed I on any civilaircraft which has a maximum certificated gross weight in excess oftwelve thousand five hundred pounds;

(D) a collision avoidance system incorporated into a transponder, andwhich produces only a signal to other aircraft, shall be installed onall civil aircraft not covered by clauses (A) and (C) above.

(2) Specialized sport, experimental, agricultural aircraft, gliders, andaircraft operating outside national airspace, as defined by theAdministrator, shall be excepted from the provisions of this subsectionwhen operating outside congested terminal airspace.

As used herein and as conventionally understood, bearing angle is meantto define the angle from the collision avoidance system equippedaircraft (own aircraft) and azimuth angle is.meant to define the anglefrom the ground SSR, to either the own aircraft or the other aircraft.

SUMMARY OF THE INVENTION It is therefore an object of the presentinvention to provide apparatus for determining passively the range andbearing angle to another mobile vehicle within a selectable proximity toone's own position from interrogation replies of the other vehiclestransponderin a secondary radar system. t

It is also an object of the present invention to provide such apparatusfor determining passively the altitude, range and bearing to a mobilevehicle possessed merely of a transponder from the replies of suchtransponder and the azimuthal or radial lines of position to such atransponderfrorn the dispersed ground stations of the radar system.

These and other objects of the present invention are accomplished by acollision avoidance system installed in an own aircraft which includesline of position circuitry for determining the azimuthal lines ofposition to the own aircraft and to another aircraft from interrogatingground stations and which includes time of arrival circuitry fordetermining the times of arrival at the own aircraft of the transponderreplies by the other aircraft to the interrogating signals emitted bythe ground stations and received by both aircraft.

From such data, the system calculates the bearing angles to the actualand imaginary locations of the other aircraft The signals representativeof the bearing angles to such actual and imaginary locations of theother aircraft are supplied to truth table circuitry wherein a decisionis made as to which signal represents the bearing angle to the actuallocation of the other aircraft and wherein the signal representative ofthe bearing angle to the imaginary location is eliminated. The slantrange is then computed from a predetermined relationship between thetime of arrival of a reply from a selected one of the ground stationsand the associated bearing angle.

In a preferred embodiment of the invention for an environment where twoground stations interrogate the own aircraft and the other aircraft, thebearing angles to the two possible locations of such other aircraft arecomputed in accordance with the following equations:

where 6, and 6 are the two possible bearing angles, Ax is the angulardifference between the two azimuthal lines of position to the aircraft,and T, and T are the times of arrival of the replies from the otheraircraft to the interrogating signals of the two ground stations.

In a preferred embodiment of the invention for a three or more groundstation environment, the bearing angles to at least four possiblelocations of such other aircraft are computed in accordance with theforegoing equations. In such case, the parameters At, T, and T, willvary in accordance with the identity of the ground stationsinterrogating the aircraft. i For a two ground station environment, thetiming sequences of the associated interrogation and reply signals arecompared in lead/lag circuitry to develop signals. against which thesigns of the calculated bearing angle signals are compared in the truthtable to determine which of the computed bearing angles represents thebearing angle to the actual location of the other aircraft. For a threeor more ground station environment, the truth table comprises circuitryfor comparing the multiple bearing angles as measured from a selectedazimuthal line of position to determine coincidence thcrebetween. Suchcoincidence permits the selection of the bearing angle to the actuallocation of the other aircraft Once the bearing angle is known. theslant range to the other aircraft is calculated according to thefollowing equation:

where a is one-half the sum of the azimuth angles (ar 01,) from thefirst SSR ground station to the own aircraft and the other aircraft, Bis one-half the sum of the azimuth angles (B 13,) from the second SSRground station to the own aircraft and the other aircraft,

T, and T are the times of arrival, C=Asina-BsinBandD=Acosa-BcosB.

The correct bearing angle, 6,, or 0 is then detected by a truth table.With the correct bearing angle known, the slant range is calculatedaccording to the following equation:

R= or I A cos (0-01)] BRIEF DESCRIPTION OF THE DRAWINGS In the Drawings:

FIGS. 1A, 1B, 1C and 1D are geometrical diagrams useful in understandingthe derivation of bearing angles in accordance with the presentinvention;

FIG. 2 is a geometrical diagram useful in explaining the preferredembodiment of a collision avoidance system shown in FIG. 3;

FIG. 3, which includes sections 3A and 3B, is a block diagramillustrating one preferred embodiment of a collision avoidance systemarranged according to the present invention;

FIG. 4 is a geometrical diagram useful in explaining the preferredembodiment of a collision avoidance system shown in FIG. 5; and

FIG. 5, which includes sections 5A and 5B, is a block diagramillustrating a second preferred embodiment of a collision avoidancesystem arranged according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the standard ICAO secondarysurveillance radar system, an SSR ground station repeatedly transmitsinterrogations at a frequency of 1030 MHz on a continuously rotatingbeam. The beam, which is conventionally controlled by the strength ofside lobe suppression signals to a width of 4, scans the surroundingarea in the clockwise direction, completing one revolution in a periodof approximately 4 to ID seconds. In the United States, the SSR groundstations are installed in a random pattern across the country, thedensity of which generally varies in accordance with the density of airtraffic.

All commercial transport aircraft and nearly all other aircraft thatutilize major airports are equipped with transponders which reply tointerrogations received from SSR ground stations. As each SSR beam scanspast an aircraft, it interrogates that aircraft transponder from aboutto 25 times at precise intervals which are indigenous to each station,e.g., 2 to 5 msec. Each interrogation initiates a reply transmissionfrom the transponder at a frequency of 1090 MHz.

The transponder reply message includes two socalled framing pulses"which are spaced apart in time by 20.3 microseconds. The intervalbetween the framing pulses contains a number of discrete sub-intervals,in each of which a pulse may or may not be transmitted, depending uponwhat information is to be contained in the reply. Twelve suchsub-intervals are available, permitting 4,096 different binary codegroups, each representing one or more pieces of information such asidentity, barometric altitude, distress signal, and so on. The desiredreply code group, now internationally standardized, may be set into thetransponder by the operator of the aircraft using manual code wheelswitches, or in some cases automatically by an altimeter or evensemiautommatically, for example by pressing a button.

Conventionally, each aircraft transponder is interrogated by each groundstation to alternately transmit the identity (A mode) and the altitude(C mode) of its aircraft. The replies to these alternate interrogationscan be decoded at the SSR ground station and utilized to place bothidentity and altitude on the radar display of the ground controlleradjacent the spot which represents the aircraft that is transmitting thereplies. A common radio channel allows the airborne transponder to replyto all ground interrogators within line of sight.

The first framing pulse (F, of the reply of a transponder follows theend of a received interrogation by a standard delay of 3 usec. Thesecond framing pulse (F is transmitted 20.3 usee after the first framingpulse. The transponder is then automatically disabled for an interval ofabout 45 to I25 psec. called the dead time.

Unlike other proposed collision avoidance systems, the system of thepresent invention is fully compatible with the present United States and[CAD secondary surveillance radar system and utilizes the I030 MHzinterrogations and the 1090 MHz replies thereto to determine passivelythe bearing angle and slant range between closely spaced aircraft.Moreover. the present system meets the requirements of the foregoingproposed legislation which would mandate that a collision avoidancesystem be installed on all air carriers, all public aircraft and largecivil aircraft, and that such systems be designed to respond to signalsautomatically generated by the transponder in any small civil aircraftnot required by the legislation to have a collision avoidance system.

A discussion first of FIGS. 1A, 1B 1C and the mathematical basis forpassive range measurements is useful to an understanding of the presentinvention. Referring thus to FIG. 1, the own station is shown at theorigin, while a single wavefront is shown travelling in the y directionfrom a very distant radar. The front reaches simultaneously the ownstation and another possible station T, units to the right of the ownstation. At this instant, a signal is initiated from the other stationand travels to the own station, arriving at a time A! later. Thedifference between the time of arrival (TOA) of the wavefront and thetime of arrival of other stations signal corresponds to the distance T,.

In order to determine the locus of all points having the same TOAdifference (T,), FIG. 1A illustrates the case when the radar wavefrontarrives at the own station first. The point(O. T l2l on the y axistypically satisfies this condition.

Considering then a point (R, 0) for the other station as indicated, thefront must travel a distance R cos 0 before it arrives at the otherstation because the wavefront arrives at the own station first. Thus,

This is the locus of a parabola with the focus at the origin.

FIG. 18 illustrates the case when the wavefront arrives at the unknownother station first. In this case,

R (-R cos0) T,

R l cos 6 Thus. a unique parabola exists for both cases.

FIG. 1C illustrates two wavefronts, one originating from a distant radarwith an azimuthal angle of 0 and with a TOA difference T and the otherwavefront originating from second radar with an angle of (1 and a TOAdifference T The two parabolas intersect at only two precise pointswhich may be determined as follows:

T R mm-parabola 2 1, T. Thl'refme l cos 9 l cos (6 a) T,+T cos6a+Tsin0sina=T +T cos cos 0 (T cosa T +sin 6 (T sin a)=T6 T if d d) 0leading Conversely, d) d) 0 lagging d) azimuth of unknown station fromradar bi) azimuth of our station from radar.

0 T (l cos a) tan 2 T sina tan NI c:

T, sin a t W T, sin? a I? sin a-l-ZTJ; (I cos a) T. (l cosa) 2Tsin a1/41. sin 1: -4T. l-cos a) 'r, H-cos a) 21,1

sin

sin

The foregoing equation l defines from the own station the two angles ofthe two intersection points of the parabolas.

In order to determine which of the two angles pertains to the positionof the unknown station, resort must be made to the slight angulardifference between the radar signal to the other station and to the ownstation.

If the known station's azimuth from the radar is greater than theazimuth to the own station. let it be called leading. Mathematically,the unknown station's deviation may be specified:

leading a 0 01+ l80 lagging 50 Condition 2 [80 0 a 360' In summary then,the bearing angle to the real location of the unknown station isdetermined as follows: First, the two 0 values are computed inaccordance with equation (1). Then. for radar l, a determination is madeas whether the unknown station is leading or lagging. For the properdeviation, a selection as to which of the computed bearing anglessatisfy the limits of condition. If only one 0 satisfies condition I,the correct 0 has been found. Condition 2 is not then required. If both0's meet condition I, the process is repeated for condition 2. Only thecorrect 9 will satisfy condition 2. With the proper 6 determined.substitution in the basic 65 parabola l R l cos will give the range,

FIG. ID illustrates the general case with radars emitting from azimuthsa, and 0: as shown with TOA differences T, and T respectively.

First rotate the angular coordinates as follows:

LetB=6a,

Thus, when 0 01 ,8 0, the foregoing analysis applies with [3 usedinstead of 0.

FIG. 2 illustrates the geometrical relationships between the signals inan airspace occupied by a reference or own aircraft incorporating acollision avoidance system, referred to as an A type aircraft and anunknown aircraft a plane within a preselected proximity to the Aaircraft and without such a system, referred to as a D type aircraft. Apair of secondary surveillance ground stations 10 and 12, SSR-l andSSR-2, respectively, repeatedly emit interrogations, including standardomnidirectional P and P suppression pulses indicated by the linesdesignated LOP-l and LOP-2, respectively, at a frequency of I030 MHz ona beam rotating in a clockwise direction. The interrogations arereceived at different angles and different times by the A aircraft andthe D aircraft equipped only with a conventional SSR transponder.Generally, the air-to-air separation between the aircraft issignificantly smaller than the distance between the aircraft and theinterrogating ground stations so that the angles at which the A and Daircraft receive interrogations are virtually the same. In the FIG. 2diagram, therefore, the lines of position (LOPs) to the two aircraftfrom each ground station can be considered parallel.

with respect to the angular relationships between the signals. a, is theazimuth angle at which the A and D aircraft receive the 1030 MHzinterrogations from SSR-l as measured from the magnetic north of suchground station. The magnetic north reference is available in theaircraft and is independent of the heading of the aircraft. a, is 0 inFIG. 2. Likewise, a is the azi muth angle at which the A and D aircraftreceive the 1030 MHz interrogations from the ground station SSR-2 asmeasured from its magnetic north. a is 90 in FIG. 2, 0: equals 01 a, or90.

In accordance with the present invention, parabolic contours T and T areshown as representative of the two times of arrival for the replysignals generated by the D aircraft. The assumption ofa parabolic curvesurrounding one of the foci of an ellipse (A aircraft) is acceptable aslong as the major axis of the ellipse is several times greater than theminor axis. Of course, as explained hereinafter, such assumption neednot be made and the actual elliptcal contours for the times of arrival Tand T may be used. Likewise, the differences between the azimuthal linesof position to the A and D aircraft may be computed and thus used incomputing bearing angles. The mathematical expression, however, isrendered more complex. The assumptions here then as to time of arrivalcontours and azimuthal lines of position are made principally tosimplify the explanation of the present invention.

As explained above, the curves T and T intersect at two points, onerepresenting a true crossing (a real D aircraft location) and the otherrepresenting a false crossing (an imaginary D aircraft location). Suchunderstanding is necessary in as much as the transponder replies of theD aircraft to the interrogations by SSR-l and SSR-2, as detected by theA aircraft, will permit the developement of two time of arrival (TOA)values which will locate the D aircraft in two locations, the real andimaginary. Applicant's invention distinguishes the two locations andestablishes the correct one, as will be explained hereinafter.

0A, is the bearing angle measured at the A aircraft from LOP-l to thefalse crossing, while 08, is the hearing angle measured at the Aaircraft from LOP-l to the true crossing. Similarly, 0A and 68 are thebearing angles measured from the A aircraft relative to LOP-2 to thefalse and true crossings, respectively. Given the magnetic northinformation, the lines of position of SSR-l and SSR-2 to the A and Daircraft and the times of arrival of the reply signals, the applicant,by his in vention, determines the true bearing angle from the A aircraftto the D aircraft and the range between them. The preferred apparatusfor carrying out such passive determination is shown in FIG. 3.

Referring then to FIG. 3. the collision avoidance system of the presentinvention installed in ones own aircraft (an A, B or C type aircraftusing the legislative denominations) includes a first receiver 20 tunedto receive RF transmission signals at I030 MHz and a second receiver 22tuned to receive signals at I090 MHz. Relating then the system of FIG. 3to the FIG. 2 diagram, the 1030 MHZ receiver 20 is adapted to receivethe SSR-l and SSR-2 interrogations which trigger a transmitter to emitI090 MHz reply messages in response thereto. The I090 MHz receiverreceives and decodes the transponder replies of the D aircraft,including the D aircraft's identity and altitude. Since it is not a partof the present invention, there is not shown in FIG. 3 (or FIG. 5)apparatus for monitoring an azimuth sector wider than the rotating mainbeam of the radar. Such apparatus (which will be utilized) is shown inthe Litchford U.S. Pat. No. 3,735,408 and the disclosure of this patentis incorporated herein by reference.

In order to compute the lines of position (LOP-l and LOP-2) from the Aaircraft to SSR-l and SSR-2, respectively, the system comprises a pairof pulse repetition frequency (PRF) selectors 24A and 248 whichsimultaneously tune" the system to the unique pulse repetitionfrequencies of SSR-l and SSR-2 ground stations and no other. A typicalPRF selector, as well as the circuitry for determining the lines ofposition and modifying a standard SSR to generate North pulses are fullydescribed in my U.S. Pat. No. 3,757,324, and thus need not be describedin detail herein. Briefly, however, the lines of position to SSR-1 andSSR-2 are comuputed by North pulse decoders 26A, 26B, interrogationdecoders 28A, 28B, interval timers and buffers 30A, 30B, beam rotationperiod timers and buffers 32A, 32B and ratio circuits 34A, 34Brespectively. As an alternative to the method for generating Northpulses described in my patent, SSRs may be simply modified to transmitomnidrectionally three pulses, the normal P the normal P and P A, and P8 spaced two microseconds apart each time the scanning beam rotatesthrough magnetic north. This triad of pulses could be encoded into asingle omnidirectional burst of north pulses by any number of knownways.

The operation of the A and B circuitry to calculate the lines ofposition (LOP-l and LOP-2) to SSR-1 and SSR-2 is the same so that theoperation of only one need be described. Thus, the A circuitry producessignals representing the line of position to the fixed ground stationSSR-l as measured from its magnetic north (01,) in the following manner:whenever the L030 MHZ receiver receives a burst of several north pulsetriads, the burst envelope is detected by the decoder 26A and a signalis applied to the timers 30A and 32A to turn them on. Some time later,when the beam from SSR-l scans the A aircraft, the beam interrogationsare detected by the decoder 28A and a signal is applied to the timer 30Ato cause the transfer of an accumulated time value to its output buffer.The time value accumulated by the timer 32A is transferred to its outputbuffer upon receipt of the next north pulse burst, i.e., every completerotation by the beam. The respective buffers supply these time values tothe ratio circuit 34A which retains such values until new time valuesare computed and the buffers updated. SSRs rotate at different angularvelocities requiring circuitry to accommodate any rotational period bythe scanning beam.

Thus, the ratio circuit 34A determines the ratio between the signalreceived from the timer 30A and the signal received from the timer 32Ato produce an output signal of proper scale, representing the angle a,in degrees. As an example, if the beam of the SSR-l ground station towhich the circuit is tuned rotates 360 in 4 seconds, and if it takes 500milliseconds from the time the North pulse burst is received until thebeam passes over the A aircraft, the angle a, will be computed as(360/8) or 45". The circuits 26B 34B operate in the same manner todetermine the line of position (01 to the SSR-2 as measured frommagnetic north.

in order to determine the angle between the lines of position to theSSRs, the signals representing the angles a, and or, are passed alongcorrespondingly labelled conductors to a combining circuit 36 where thevalues of the two signals are subtracted from each other. The outputsignal, denoted 01 represents the angular difference clockwise betweenLOP-l and LOP-2 using LOP-l as the reference.

Currently with the determination of the lines of position to SSR-l andSSR-2, the times of arrival at the A aircraft of the transponder repliesof the D aircraft are determined (FIG. 2). To this end, the collisionavoidance system of the present invention comprises a pair of PRFselectors 40A and 408 which, like the selectors 24A and 24B, respondonly to the interrogations of the SSR-l and SSR-2 ground stations. Theinterrogation signals are passed on to the input terminals of a pair ofgates 42A and 428 which are enabled by the P pulses of the L030 MHZinterrogations. At the same time, the P pulses are supplied to a pair ofinterval timers 44A and 448 to clear the timers and turn them on.

The transponder reply signals of the D aircraft detected by the L090 MHzreceiver 22 are supplied to a pair of reply, altitude and identitydecoders 46A and 468 which decode the replies and supply them to theother input terminals of the gates 42A and 423. In addition, the replydecoders respond to the mode C replies that indicate the intruderaircraft is within a predetermined common altitude band. As disclosed inmy US. Pat. No. 3,735,408, the decoders decode the altitude mode Creplies to ascertain the altitude of the other aircraft and also producesignals representative of its own aircrafr. The signals are compared toproduce common altitude stratum signals if the altitudes are with aselected range, e.g., $2000 feet. Also, the decoders 46A and 46B decodethe mode A signals to identify the other aircraft. Although not shown,the decoders 46A and 468 may be tied together through a comparator inorder to assure correspondence between the identity of the reportingother aircraft. In this way, replies from aircraft beyond a certaincommon altitude stratum or from different aircraft will be discarded. Inturn. the gates enabled by the P pulses of the associ atcdinterrogations, supply the replies to the interval timers 44A and 448 toturn the timers off. Thus, the output signals developed by the timers44A and 44B represent the times of arrival of the reply signals at theown or A aircraft generated by the transponder of the D aircraft inresponse to interrogations by the SSR-l and SSR-2 ground stations. Theconversion of TOA values representing the parabolas may be made bydividing the time values by 6.1838 microseconds since radiation travelsI NM in 6.l838 microseconds. With such conversion, the time of arrivalvalues in terms of geographic parabolas, designated herein as T, and T(corresponding to SSR-l and SSR-2, respectively), are determined. The T,and T signals are carried along correspondingly labelled conductors.

The system further includes a pair of lead/lag circuits 47A and 47Bwhich develop either negative or positive voltage levels in response tothe timing sequence of the 1030 MHz interrogation signals and thereception of the 1090 MHz reply signals. To this end, the reply signalsas detected by the L090 MHz receiver 22 are also supplied to the inputterminals of the circuits 47A and 47B and the interrogation signals, asdetected by the selectors 40A and 40B are supplied to other inputterminals of the circuits 47A and 47B.

As explained in my US. Pat. No. 3,757,324 wherein a typical lead/lagcircuit is described and shown (FIG. 7), all SSR beams rotate clockwiseas viewed from above. When the SSR-l beam illuminates the A or ownaircraft carrying the apparatus of FIG. 3 before illuminating the Daircraft the circuit 47A generates a positive voltage level.Alternatively, when the D aircraft is illuminated by the SSR-l scanningbeam before the A aircraft, which is the case in FIG. 2, the circuitsupplies a negative voltage level. Similarly, the lead/lag circuit 47Bsupplies a positive voltage level when the SSR-2 beam illustrates the Aaircraft before the D aircraft, which is the case in FIG. 2, and anegative signal when the SSR-2 beam illuminates the D aircraft beforeilluminating the A aircraft. The plus or minus signals developed by thelead/lag circuits form part of a truth table, explained hereinafter,which resolve the ambiguities resulting from the dual crossings of thewavefronts T, and T The plus or minus voltage level signals developed bythe circuits 47A and 47B are carried by conductors 48A and 488,respectively. It is noteworthy that in addition to the lead/lag data,the azimuthal separation between the A and D aircraft relative toselected SSR stations may be determined. This angular difference(assumed to be zero in the FIG. 2 diagram) and its polarity is utilizedin mathematical solution for determining bearing angle and range givenhereinabove.

In accordance with the present invention, during .each sweep by thebeams of SSR-l and SSR-2, the signals representative of the times ofarrival T, and T and the signal (a) representative of the angulardifference between the lines of position are supplied to a logic circuit49 wherein the bearing angles 6A, and 6B, are computed in accordancewith the following formulae:

I a Sin 2 The foregoing equations determine the two bearing angles fromthe A aircraft to the two intersection points of the parabolicwavefronts shown in FIG. 2. Thus, 6A, is the bearing angle from the Aaircraft to the false position of the D aircraft using LOP-l as areference. 08, is the bearing angle from the A aircraft to the actualposition of the D aircraft using LOP-1 as a reference. The computationof the A, and 9B, angles may be carried out in conventionalmicrocomputer circuits such as those incorporated in the widely usedHewlett-Packard HP-35.

The signals representative of the computed bearing angles 0A, and 0B,are carried along correspondingly designated conductors to combiningcircuits 50A and 50B and to a pair of sign detector circuits 52A and528. In the combining circuits 50A and 50B, the signal representative ofthe angular difference between LOP-l and LOP-2 is subtracted from the6A, and 6B, signals to develop signals representative of bearing angles0A and 6B The 0A and 0B, signals are likewise supplied alongcorrespondingly labelled conductors to a pair of sign detector circuits54A and 54B. In the sign detector circuits 52A, 54A, 52B, 548 thesignals are compared against a fixed reference signal representing 180to develop positive signals in the event the supplied angles are lessthan 180 and negative signals in the event the supplied angles aregreater than 180. Thus, the four outputs from the sign detector circuits52A, 54A, and 52B, 548 will become combination of positive and negativevoltages which contain the required information to permit the selectionof the bearing angle to the actual location of the D aircraft.Significantly, the imaginary location or ambiguous second crossing ofthe two parabolas is identified as false and eliminated.

The circuitry for eliminating the false crossing and determining thetrue bearing angle comprises a comparator circuit 56A to which the signof the 0A, signal is supplied, a comparator circuit 58A to which thesign of the 6A, signal is supplied, a comparator circuit 56B to whichthe sign of the 0B, signal is supplied and a comparator circuit 588 towhich the sign of the BB, signal is supplied. The other input terminalsof the comparators 56A and 56B are supplied with either positive ornegativie voltage levels from the lead/lag circuit 47A. In a likemanner, the comparator circuits 58A and 58B are supplied with positiveor negative voltage levels developed by the lead/lag circuit 478.

As described above, the lead/lag circuits 47A and 47B develop positivevoltage levels in response to the early detection of the 1,030 MHzinterrogation of the A aircraft and negative voltage levels in responseto the early detection of the L090 MHz replies from the D aircraft. Thecomparators are designed to operate in response to preselectedpolarities appearing at their input terminals. Thus for instance, thecomparators 56A and 58A respond only to either two positive or twonegative input signals to supply positive output signals along aconductor leading to an AND gate 60A, referred to as the BA, truth gate.Comparators 56B and 58B are likewise designed to respond to either twopositive or two negative input signals to supply positive output signalsalong conductors leading to an AND gate 608, designated as the 0B, truthgate. When signals of opposite polarity are supplied to the input terminals of the comparators 56A. 58A and 56B. 58B, the comparators supplynegative output signals.

The gates 60A and 60B which may be of typical AND" gate constructionrespond to positive input signals of equal amplitude to supply enablingsignals along conductors leading to a pair of transmission gates 62A and628. The other input terminals of the transmission gates 62A and 62B aresupplied with the signals representative of the bearing angles 0A, and6B,, respectively. Depending upon which of the truth gates 60A or 608 isenabled, the transmission gate 62A or 628 will transmit the true bearingangle, be it 6A, or 98, to a logic circuit 64 wherein the range betweenthe A and D aircraft is calculated.

The circuit 64 calculates the range (R) in accordance with the followingequation:

l cos 0 where 6 is the angle selected from either 6A, or 0B, and T, isthe time of arrival of the reply signal from the D aircraft generated inresponse to interrogation by the ground station SSR-l. The signalrepresentative of range is then supplied to an indicator 66 where therange is displayed and to a range rate indicator 68 where the range rateis displayed.

6A, and B, are the bearing angles as measured at aircraft A relative tothe line of position from SSR-l to the D aircraft. In order for aircraftA to compute the bearing angle with respect to magnetic north. theoutput terminals of the transmission gates 62A and 62B are also suppliedto a logic circuit 70 where the signals representative of such bearingangles are combined with the signal representative of the angle a, todevelop a signal representative of the true bearing angle. This angle isthen displayed by a bearing angle indicator 72.

It is well known that range and range rate can be combined to obtain thevalue TAU, or time to collision. if the TAU value is small enough, acommand signal is generated which directs the pilot to climb or descendto avoid the approaching aircraft. Bearing angle, range rate and rangemay be combined to restrict the generation of command signals to thoseinstances where there is a real possibility of a collision.Specifically, the transverse velocity of the other aircraft may becalculated from such signals. If the velocity is increasing, the TAUcircuitry will be inhibited. If the velocity is decreasing, the TAUcircuitry will be enabled and the pilot directed to take correctiveaction immediately. The TAU logic is described in detail in the AirTransport Association's report entitled ANTC-l 17''.

With the airspace and ground configuration as that shown in FIG. 2 where0: equals and assuming T, 2 and T 3, 0A, and 013, may be calculated asfollows:

V l l a sin;

With the foregoing voltage configuration, comparators 56A and 58A willbe disabled; comparators 56B and 583 will be enabled. The B truth gate608 will likewise be enabled and the value of 6B,, the true bearingangle transmitted to logic circuit 64. False bearing angle 0A, iseliminated. The range R may then be calculated as follows:

R: l+cos222 The collision avoidance system of FIG. 3 utilizes theinterrogation of a pair of SSR ground stations and the D aircraftsreplies thereto to compute the range and bearing angle from the Aaircraft to the D aircraft. When more than two ground stations areavailable, the computation is technologically simplified.

Thus, referring to FIG. 4, there are shown three fixed ground stationslabelled SSR-l, SSR-2 and SSR-3, respectively, which interrogate an Aaircraft and a D aircraft located in a selected proximity to the Aaircraft. The A aircraft is shown as a triangle, while the D aircraft isshown as a circle. The lines of position to the A aircraft are labelledLOP-l, LOP-2 and LOP-3. Parabolic contours for the TOA signals asmeasured by the A aircraft are labelled T,, T and T respectively. Asexplained hereinabovoe, the contours have approximately parabolic shapes(actually elliptical), with each of the parabolas intersecting at twopoints, a true location and a false location of the intruder or Daircraft.

The intersections of the parabolic wavefronts T T and T at the truelocation of the D aircraft are labelled A, C and E; A representing thecrossing T T C representing the crossing T,-T and E representing thecrossing T T The false crossing of the two parabolas T, and T isindicated by the letter B, the false crossing of the parabolas T, and Tis indicated by the letter D and the false crossing of the parabolas Tand T is indicated by the letter F.

Also shown are four separate arcs, indicating the angular bearing of theA aircraft to the true and false crossings as measured from LOP-1. Theangles thus to the true crossing are 6A,, 0C, and 0E,. The angles, asmeasured from LOP-1 to the false crossings B, D and F are labelled,respectively, 08,, 0D, and 01 The collision avoidance apparatus shown inFIG. utilizes the lines of position to the ground station SSRs and thetimes of arrival of the three transponder replies to these same SSRs ofthe interrogated D aircraft to calculate the bearing angle and slantrange to such D aircraft.

Hence, the system includes a L030 MHZ receiver and a 1,090 MHz receiver82. Also provided are line of position (LOP) circuits 84A, 84B and 84C,exemplary configurations of which are shown in FIG. 2 and describedhcreinabove, which calculate the azimuthal lines of position 11,, a anda from the A aircraft to SSR-l, SSR-2 and SSR-3 ground stations.respectively. in order to compute the times of arrival at the A aircraftof the replies generated by the transponder equipped D aircraft inresponse to the interrogations by the three SSRs, time of arrivalcalculation circuits 86A, 86B and 86C are identified by the letters T Tand T respectively. The conductors to which the signals are supplied areidentified by the same letters.

The a and a signals are supplied to a combining circuit 88 wherein the asignal is subtracted from the 0: signal to provide an angular differencesignal ax. Similarly, the a, signal is subtracted from the (1 signal ina combining circuit 90 to provide as an output the angular differencesignal cry.

The angular difference signal 00: and the signals representative of thetimes of arrival T and T are supplied to logic circuit 92 wherein theangles 6A and 6B, are computed in accordance with the followingformulae:

V 2] 0/1, 2 tan* cot 2 ax sin at vT lT 68, 2 tan cot L sin 2 sin Asshown in FIG. 4, only the wavefront pairs associated with SSR-l andSSR-3 and with SSR-l and SSR-2 are utilized in the computation. Thepairs associated with SSR-2 and SSR-3 may also be used, in which eventthe angular values 0E, and GF, would be calculated. However, as long asthere is sufficient spacing between SSRs and, accordingly, asufficiently large angular difference between lines of position, twopairs will suffice, be they between SSR-l and SSR-2, SSR-l and SSR-3 orbetween SSR-2 and SSR-l, SSR-2 and SSR-3. In any event, the line ofposition associated with the two largest times of arrival should be usedas the standard of comparison.

The signals representative of the computed angles BA BB BC, and 0D, arethen supplied along correspondingly labelled conductors to fourcombining circuits 96, 97, 98 and 99 where the angles representative oftrue and false crossing points are subtracted. Thus,

6C is subtracted from GA. in circuit 96, D. is subtracted from GA. incircuit 97. 6C. is subtracted from 8D in circuit 98 and 0D, issubtracted from 68, in circuit 99. As will be apparent from FIG. 4. themagnitudes of the differences will be zero (theoretically) and in theleast minimal at the true crossing point (A. C. E) and large at thefalse crossings (B. D. F). Thus. combining circuits 96, 97, 98 and 99determine that two of the four angles are equal and that the angulardifference between them is zero or a very small value.

Signals with magnitudes representative of the angular differences asdetermined by the circuits 96, 97 and 98, 99 are then supplied to a pairof selector circuits 100 and 102. respectively. which transmit only thesignals with the smallest magnitudes. With correspondence between theangles OA, and BC the selector 100 supplies a zero voltage signaloutput. while the selector 102 supplies a signal output having amagnitude representative of the smallest angular variation between theangles 6B,. 0C. and 05,. 0D,.

The smallest angular variation signals are then supplied to a comparator104 which compares the two signals and. depending on which of thesignals has the smallest magnitude supplies an enabling signal to Aenable gate 106 or an enabling signal to a 9B, enable gate 108. Logiccircuits 92 and 94 supply the (9A and 6B, signals to the other inputterminals of the gates I06 and 108, as indicated by the conductorslabelled 8A. and 63 Thus. depending upon which of the gates I06 and 108is enabled, the gate will transmit the true bearing angle. either 6A, or08. to a logic circuit 110 wherein the range between the A and Daircraft is determined. According to the present invention, the circuit110 calculates the range in accordance with the equation:

R l cos 9 where 0 will be either 0A, or 68.. The signal T, is suppliedalong the conductor labelled T to another input terminal of the circuit110 in order to implement the calculation.

From the logic circuit 110 the range representative signal is suppliedconcurrently to a range indicator 112 and a range rate indicator 114.The range indicataor displays the range. while the range rate indicator114 displays the range rate in knots. As shown in FIG. 4, 49A and 0B arethe bearing angles as measured from the line of position of the Aaircraft to the SSR-l ground station. A circuit 116 is thus providedwhich combines the angle a, which is the angle between magnetic northand LOP-1 with either 0A. or 0B. depending upon which of the gates 106or 108 is enabled. The resultant signal represents the true bearing withrespect to magnetic north.

it will be noted that the system of FIG. utilizes two or more coincidentbearing angles measured from a selected line of position. whereas thesystem of FIG. 3 utilizes lead/lag logic to separate the true crossingfrom the false crossing. Thus, as the environment of SSR ground stationschanges. either or both methods can be used to compute the true bearingangle and the true slant range to the other aircraft. in each instance.the computation is done entirely passively and the requirements of theproposed legislation met.

Although the applicant's invention has been described with respect totwo illustrative embodiments it will be understood that the invention issusceptible of modifications. Thus. for instance. in an environmentwhere the A and D aircraft are interrogated by a multiplicity of groundstations, angular selection logic may be incorporated to select thelines of position with greatest angular separation. rather than randomlyselecting the ground stations or expanding the logic to ac commodate allsuch ground stations. Also. the assumptions of parabolic contours forthe TOAs and the angular equality of the azimuthal lines of position tothe own and other aircraft are not critical. but have been made tosimplify the description of the applicants invention. The angulardifference between the azimuthal lines of position to the own and otheraircraft may be determined. ln such a case. the elliptical contour ofthe curve surrounding one of the foci of an ellipse (own aircraft) isutilized. with bearing angle and slant range being computed inaccordance with the equations given hereinabove. Also. the own aircraftmay estimate the range to the SSR ground stations. as well as to theother aircraft.

1 claim:

I. In a system for collision avoidance and/or proximity warningindication at an own location utilizing the signals transmitted byscanning beam secondary surveillance radar (SSR) stations and replymessages transmitted by a nearby transponder replying to said SSRsignals. apparatus for passively determining the bearing angle from saidown location to said transponder comprising:

a. means responsive to said SSR ground station signals for generatingsignals representative of the azimuthal lines of position from selectedradar stations to the own location and to the location of saidtransponder;

b. means responsive to selected reply messages transmitted by saidtransponder and responsive to the SSR ground station signals forproviding signals representative of the time of arrival of each of saidselected reply messages at said own location; and

c. means responsive to signals representative of selected azimuthallines of position and to the corresponding signals representative ofselected times of arrival for providing a signal indicative of thebearing angle from the own location measured from said lines of positionto the location in space from which the transponder replies originate.

2. Apparatus according to claim 1 for passively determining the range tosaid transponder from said own location comprising:

d. means responsive to one of said selected times of arrivalrepresentative signals and the signal indicative of the bearing anglefor providing a signal representing the distance separating saidtransponder from said own location.

3. Apparatus according to claim 2 wherein the own location is anaircraft and said transponder is located on another aircraft, furthercomprising:

e. means for decoding altitude reporting signals forming a part of thetransponder replies from the other aircraft;

f. means for producing similar own aircraft altitude signals;

g. means for comparing said altitude reporting signals and said similarown aircraft altitude signals to produce common altitude stratumsignals; and

h. means responsive to the presence of said common altitude stratumsignals for controlling the means for providing the bearing angleindicative signals and the means for providing the distance indicativesignals.

4. Apparatus according to claim 3 further comprising:

i. means for decoding the identity reporting signals in the transponderreplies of the other aircraft;

j. means for determining correspondence between said identity reportingsignals; and

k. means for controlling the means for providing the bearing angleindicative signals in accordance with the correspondence between saididentity reporting signals.

5. Apparatus according to claim 2 further comprising:

I. first indicator means responsive to said range representative signalfor displaying the distance separating the own location from saidtransponder; and

m. second indicator means responsive to said range representative signalfor providing signals indicative of the rate of change of said rangerepresentative signal and inclusing means for displaying said rangerate.

6. Apparatus according to claim 2 comprising:

n. third indicator means responsive to the bearing angle representativesignal for displaying the bearing of said transponder.

7. Apparatus according to claim 2 wherein the means for providing abearing angle indicative signal comprises means responsive to thesignals representative of selected azimuthal lines of position and tothe corresponding signals representative of selected times of arrivalfor providing signals indicative of the bearing angles from the ownlocation measured from said lines of position to the locations in spacefrom which transponder replies actually originate and from whichtransponder replies apparently originate.

8. Apparatus according to claim 7 wherein the means for providing abearing angle indicative signal comprises means responsive to signalsrepresentative of selected azimuthal lines of position originating fromgeographically separated SSR stations for providing signalsrepresentative of the angular differences between said selectedazimuthal lines of position and means responsive to said angulardifference signals and to signals representative of the correspondingtimes of arrival for generating said actual and apparent bearing angleindicative signals.

9. Apparatus according to claim 7 further comprising:

0. truth table means responsive to preselected relationships betweensaid bearing angle indicative signals, said truth table means comprisingmeans for identifying said signals as indicative of either the truebearing angle to the locations in space from which the transponderreplies actually originate or the false bearing angles to the locationsin space from which the transponder replies apparently originate andmeans for transmitting the signal representative of the actual bearingangle and blocking the signals representative of the apparent bearingangles.

10. Apparatus according to claim 9 wherein two secondary surveillanceradar stations interrogate said own location and said transponder andwherein the truth table means comprises means for assigning said actualand apparent bearing angle representative signals selected polarities inaccordance with their angular magnitudcs, lead/lag logic meansresponsive to the reply messages transmitted by said transponder fordeveloping signals with selected polarities in accordance with theazimuthal positioning of said transponder relative to the own location'sazimuth from the secondary surveillance radars, means for comparing saidassigned polarities and the signals developed by said lead/lag logicmeans to determine the true bearing by polarity, and means controlled bysaid comparator means for transmitting the actual bearing anglerepresentative signal and eliminating the apparent bearing anglerepresentative signals.

ll. Apparatus according to claim 9 wherein at least three secondarysurveillance radar stations interrogate said own location and saidtransponder and wherein said means for providing a bearing angleindicative signal comprises means responsive to selected azimuthal linesof position and to corresponding signals representative of selectedtimes of arrival for developing signals indicative of at least fourbearing angles as measured from the azimuthal lines of position from oneof said secondary surveillance radar stations to the locations in spacefrom which transponder replies actually and apparently originate.

12. Apparatus according to claim ll wherein the truth table meanscomprises compartor means responsive to said bearing angle indicativesignals for determining the coincidence in magnitude between two of saidbearing signals, such two signals being essentially equal andconsequently representative of the actual bearing angle, and furthercomprising means controlled by said comparator means for transmittingthe actual bearing angle representative signal and eliminating theapparent bearing angle representative signals.

13. A method for passively determining the bearing of a transponderwithin a selectable proximity to ones own position from theinterrogation replies of the transponder in the coverage of a secondaryradar system, comprising the steps of:

a. developing signals representative of the lines of position to saidown position and to said transponder position from selected radarstations interrogating the transponder and said own position;

b. developing signals representative of the times of arrival at said ownstation of each of said interrogation replies from the transponder tothe selected secondary surveillance radars;

c. establishing a preselected relationship between said lines ofposition and said times of arrival signals; and

(1. developing a signal representing the bearing angle to thetransponder in accordance with the preselected relationship between saidlines of position and times of arrival signals.

14. The passive method according to claim 13 comprising the further stepof:

e. developing a signal representing the range from said own position tothe transponder in accordance with a preselected relationship betweensaid bearing angle signal and at least one of said time of arrivalsignals.

15. The passive method according to claim 14 wherein the transponder iscarried by an aircraft and comprising the further steps of:

f. decoding the altitude reporting signals contained in said transponderreplies;

g. comparing the decoded altitude of said transponder with the altitudeof said own position; and

h. controlling the development of the bearing angle signal in accordancewith the difference between prising the further steps of:

l. displaying the distance separating the own position from thetransponder; and m. displaying the rate of change in the distanceseparating the own position from the transponder. 18. The passive methodaccording to claim 14 comprising the further step of:

n. displaying the bearing angle to the transponder. 19{ The passivemethod according to claim 14 comthe altitude of said own position andthe altitude of prising the further steps of:

said transponder.

16. The passive method according to claim 14 wherein the transponder iscarried by an aircraft and comprising the further steps of:

i. decoding the identity reporting signals in said transponder replies;

j. comparing the decoded identities of the transponder; and

k. controlling the development of the bearing angle signal in accordancewith the coincidence between the identities of the transponder.

17. The passive method according to claim 14 com- 0. developing signalsindicating the rate of change in the range between said own position andsaid transponder;

p. developing command signals corresponding to the time separationbetween said own position and said transponder; and

q. combining the range, range rate and bearing angle signals to controlthe development of the command signal in accordance with the movement bythe transponder toward or away from said own positlon.

1. In a system for collision avoidance and/or proximity warningindication at an own location utilizing the signals transmitted byscanning beam secondary surveillance radar (SSR) stations and replymessages transmitted by a nearby transponder replying to said SSRsignals, apparatus for passively determining the bearing angle from saidown location to said transponder comprising: a. means responsive to saidSSR ground station signals for generating signals representative of theazimuthal lines of position from selected radar stations to the ownlocation and to the location of said transponder; b. means responsive toselected reply messages transmitted by said transponder and responsiveto the SSR ground station signals for providing signals representativeof the time of arrival of each of said selected reply messages at saidown location; and c. means responsive to signals representative ofselected azimuthal lines of position and to the corresponding signalsrepresentative of selected times of arrival for providing a signalindicative of the bearing angle from the own location measured from saidlines of position to the location in space from which the transponderreplies originate.
 2. Apparatus according to claim 1 for passivelydetermining the range to said transponder from said own locationcomprising: d. means responsive to one of said selected times of arrivalrepresentative signals and the signal indicative of the bearing anglefor providing a signal representing the distance separating saidtransponder from said own location.
 3. Apparatus according to claim 2wherein the own location is an aircraft and said transponder is locatedon another aircraft, further comprising: e. means for decoding altitudereporting signals forming a part of the transponder replies from theother aircraft; f. means for producing similar own aircraft altitudesignals; g. means for comparing said altitude reporting signals and saidsimilar own aircraft altitude signals to produce common altitude stratumsignals; and h. means responsive to the presence of said common altitudestratum signals for controlling the means for providing the bearingangle indicative signals and the means for providing the distanceindicative signals.
 4. Apparatus according to claim 3 furthercomprising: i. means for decodIng the identity reporting signals in thetransponder replies of the other aircraft; j. means for determiningcorrespondence between said identity reporting signals; and k. means forcontrolling the means for providing the bearing angle indicative signalsin accordance with the correspondence between said identity reportingsignals.
 5. Apparatus according to claim 2 further comprising: l. firstindicator means responsive to said range representative signal fordisplaying the distance separating the own location from saidtransponder; and m. second indicator means responsive to said rangerepresentative signal for providing signals indicative of the rate ofchange of said range representative signal and inclusing means fordisplaying said range rate.
 6. Apparatus according to claim 2comprising: n. third indicator means responsive to the bearing anglerepresentative signal for displaying the bearing of said transponder. 7.Apparatus according to claim 2 wherein the means for providing a bearingangle indicative signal comprises means responsive to the signalsrepresentative of selected azimuthal lines of position and to thecorresponding signals representative of selected times of arrival forproviding signals indicative of the bearing angles from the own locationmeasured from said lines of position to the locations in space fromwhich transponder replies actually originate and from which transponderreplies apparently originate.
 8. Apparatus according to claim 7 whereinthe means for providing a bearing angle indicative signal comprisesmeans responsive to signals representative of selected azimuthal linesof position originating from geographically separated SSR stations forproviding signals representative of the angular differences between saidselected azimuthal lines of position and means responsive to saidangular difference signals and to signals representative of thecorresponding times of arrival for generating said actual and apparentbearing angle indicative signals.
 9. Apparatus according to claim 7further comprising: o. truth table means responsive to preselectedrelationships between said bearing angle indicative signals, said truthtable means comprising means for identifying said signals as indicativeof either the true bearing angle to the locations in space from whichthe transponder replies actually originate or the false bearing anglesto the locations in space from which the transponder replies apparentlyoriginate and means for transmitting the signal representative of theactual bearing angle and blocking the signals representative of theapparent bearing angles.
 10. Apparatus according to claim 9 wherein twosecondary surveillance radar stations interrogate said own location andsaid transponder and wherein the truth table means comprises means forassigning said actual and apparent bearing angle representative signalsselected polarities in accordance with their angular magnitudes,lead/lag logic means responsive to the reply messages transmitted bysaid transponder for developing signals with selected polarities inaccordance with the azimuthal positioning of said transponder relativeto the own location''s azimuth from the secondary surveillance radars,means for comparing said assigned polarities and the signals developedby said lead/lag logic means to determine the true bearing by polarity,and means controlled by said comparator means for transmitting theactual bearing angle representative signal and eliminating the apparentbearing angle representative signals.
 11. Apparatus according to claim 9wherein at least three secondary surveillance radar stations interrogatesaid own location and said transponder and wherein said means forproviding a bearing angle indicative signal comprises means responsiveto selected azimuthal lines of position and to corresponding signalsrepresentative of selected times of arrival for developing signalsindicative of at least four bearing angles as measureD from theazimuthal lines of position from one of said secondary surveillanceradar stations to the locations in space from which transponder repliesactually and apparently originate.
 12. Apparatus according to claim 11wherein the truth table means comprises compartor means responsive tosaid bearing angle indicative signals for determining the coincidence inmagnitude between two of said bearing signals, such two signals beingessentially equal and consequently representative of the actual bearingangle, and further comprising means controlled by said comparator meansfor transmitting the actual bearing angle representative signal andeliminating the apparent bearing angle representative signals.
 13. Amethod for passively determining the bearing of a transponder within aselectable proximity to one''s own position from the interrogationreplies of the transponder in the coverage of a secondary radar system,comprising the steps of: a. developing signals representative of thelines of position to said own position and to said transponder positionfrom selected radar stations interrogating the transponder and said ownposition; b. developing signals representative of the times of arrivalat said own station of each of said interrogation replies from thetransponder to the selected secondary surveillance radars; c.establishing a preselected relationship between said lines of positionand said times of arrival signals; and d. developing a signalrepresenting the bearing angle to the transponder in accordance with thepreselected relationship between said lines of position and times ofarrival signals.
 14. The passive method according to claim 13 comprisingthe further step of: e. developing a signal representing the range fromsaid own position to the transponder in accordance with a preselectedrelationship between said bearing angle signal and at least one of saidtime of arrival signals.
 15. The passive method according to claim 14wherein the transponder is carried by an aircraft and comprising thefurther steps of: f. decoding the altitude reporting signals containedin said transponder replies; g. comparing the decoded altitude of saidtransponder with the altitude of said own position; and h. controllingthe development of the bearing angle signal in accordance with thedifference between the altitude of said own position and the altitude ofsaid transponder.
 16. The passive method according to claim 14 whereinthe transponder is carried by an aircraft and comprising the furthersteps of: i. decoding the identity reporting signals in said transponderreplies; j. comparing the decoded identities of the transponder; and k.controlling the development of the bearing angle signal in accordancewith the coincidence between the identities of the transponder.
 17. Thepassive method according to claim 14 comprising the further steps of: l.displaying the distance separating the own position from thetransponder; and m. displaying the rate of change in the distanceseparating the own position from the transponder.
 18. The passive methodaccording to claim 14 comprising the further step of: n. displaying thebearing angle to the transponder.
 19. The passive method according toclaim 14 comprising the further steps of: o. developing signalsindicating the rate of change in the range between said own position andsaid transponder; p. developing command signals corresponding to thetime separation between said own position and said transponder; and q.combining the range, range rate and bearing angle signals to control thedevelopment of the command signal in accordance with the movement by thetransponder toward or away from said own position.